The present invention pertains to a device for converting rotary motion into rectilinear motion, and more specifically to such a rotary-to-linear converter of the type having a worm and a ball nut with a plurality or multiplicity of antifriction balls rollably interposed therebetween. The invention also specifically deals with a two-speed feed mechanism built on the novel rotary-to-linear conversion scheme.
The combination of a worm and a ball nut with interposed steel balls has found extensive use for feed motions and other purposes. The balls convert sliding friction into rolling friction and thus afford smooth back-and-forth travel of either of the worm and the ball nut with the bidirectional rotation of the other or of the same part. As heretofore constructed, however, the worm-and-ball-nut assembly has had one drawback arising from the movement of the balls. The conventional principle has been to "recirculate" the balls; that is, they return from each end of the ball nut to the other through a tubular return guide after making, for instance, one and a half to three turns around the worm. Such recirculation of the balls gives rise to considerable noise.
Another objection to the prior art concerns the backlash between the worm and the ball nut, due to the presence of axial clearances between worm and balls and between ball nut and balls. The backlash is a common cause of (1) the uneven wear of the relatively moving parts, (2) noise, and (3) a delay with which either of the worm and the ball nut starts moving axially in response to the rotation of the other. It has been suggested for the elimination of the backlash to split the ball nut into two halves and to place a spacer therebetween. When mounted in place, the ball nut assembly is preloaded in a direction of either axial compression or expansion. This known solution is still unsatisfactory because the ball nut assembly must be of sufficiently sturdy make to withstand the stresses exerted thereon at the time of mounting. The preloading of the ball nut assembly is also in itself a trouble.
The conventional worm-and-ball-nut assembly has the additional disadvantage of admitting ready intrusion of dust and other foreign matter into the ball-receiving space between worm and ball nut from both ends of the latter. Accumulating on the threaded surfaces of the worm and the ball nut, such foreign matter impedes the smooth rolling of the balls and, in consequence, the smooth relative motion of the worm and the ball nut.
The worm-and-ball-nut assembly in accordance with the invention finds typical applications in the feed mechanisms of machine tools and other pieces of machinery. In such applications the need often arises for rotatably mounting the ball nut to some fixed part. The usual conventional practice to this end has been to place between the ball nut and the fixed part an antifriction bearing comprising two concentric rings and rolling elements, normally balls, therebetween. The use of the separate bearing is objectionable because of the additional installation space required.
A variety of mechanisms have been proposed and used for feeding, for example, cutting tools, worktables or work itself at extremely low speed and moving them back at higher speed on a return stroke. Examples include step pulley arrangements with or without gears, variable ratio gear trains, differential gearing, and variable ratio friction drives. These known feed mechanisms are mostly bulky, complex in construction, and not necessarily well adapted for feeding a desired object in a manner suitable for precision tooling.